It is conventional to provide a switch assembly to receive mechanical gear selection inputs from the driver of a vehicle through various linkages and output electrical signals to the transmission electronic control unit indicative of the gear selection. Such switch assemblies are coupled to the detent lever and the manual valve of the transmission and generally comprise either a rotary movable contact mechanism such as those shown in U.S. Pat. Nos. 5,527,768 and 5,440,087, assigned to the assignee of the present of the invention, or a straight linear movable contact mechanism such as that shown in U.S. Pat. No. 5,231,254, also assigned to the assignee of the present invention. The present invention relates to the latter type in which an actuator carrying a set of movable contacts is caused to slide back and forth in a straight line in dependence upon the longitudinal movement of the manual valve. The movable contacts are spring biased into engagement with a stationary contact board having contact segments arranged in a selected pattern so that on/off output signals are obtained based on the longitudinal position of the manual valve. The housing of the switch mechanism is open to the passage of transmission fluid including contaminants into and out of the switching chamber.